1. Field of the Invention
This invention relates to processes for the conversion of the sugar galactose to the isomeric sugar tagatose. The invention also relates to production of tagatose from lactose, whey, or deproteinized whey as raw material and to the recovery of tagatose from mixtures of sugars.
2. Description of the Related Art
Lobry De Bruyn et al, Recueil des Travaux Chimiques des Pays-Bas (1897) 16:262-273, describes the isolation of D-tagatose in low yield from the complex mixture obtained by treating D-galactose with alkali. Reichstein et al, Helv. Chim. Acta, (1934), 17:753-761, disclose the isomerization of D-galactose to D-tagatose using pyridine. Pavlouska et al, Czechoslavakia Patent No. 142,959, Mar. 31, 1971, disclose the manufacture of D-tagatose using pyridine. These processes were limited in yield by the unfavorable equilibrium which favors galactose. Hicks U.S. Pat. No. 4,273,922, June 16, 1981, discloses the addition of boric acid to aldose sugars in the presence of tertiary or quaternary amines. Boric acid complexes the ketose as it is formed and effectively shifts the equilibrium. This procedure yields about 50% D-tagatose. Removal of the boric acid after completion of the reaction is difficult and requires an expensive specialty ion exchange resin. Szeja et al, Poland Patent No. 113,487, May 15, 1952, disclose the preparation of D-tagatose from D-galactose using 1,3-dicyclohexylcarbodiimide as an isomerization and complexation agent. Kubala et al, Czechoslovakia Patent No. 221,039, Sept. 15, 1982, discloses the preparation of D-tagatose from galactose by isomerization with a strongly basic anion exchange resin in the hydroxyl form. Barker et al, European Patent Application EP0109203, May 23, 1984, disclose the isomerization of aldoses to ketoses under acidic conditions with calcium chloride. Kubala, Czechoslovakia Patent No. 8402745-A, 1985, discloses the isomerization of L-galactose to L-tagatose with ion exchangers. All of the prior art processes appear to be unsuitable for mass production of tagatose.